Through the proliferation of portable equipment and appliances, the demand for rechargeable batteries is rising by the day. This has brought about innovations and improvement in rechargeable battery design. The most commonly used secondary battery used in portable applications is the Nickel Cadmium (NICD) battery.
Although newer NICD batteries offer higher energy densities, the demand for more power and longer operation time is outstripping the pace of its progress. There is also an increasing concern about contamination of the environment during disposal of spent NICD cells due to the toxic nature of its Cadmium content. These created a pressing need for a new kind of battery to replace the NICD cell. This battery should have a higher energy storage density than the best NICD designs but should not possess any harmful content in its composition that can hurt the environment during disposal (i.e. easily disposable).
The new Nickel Metal Hydride (NIMH) battery provides a viable alternative to its NICD counterpart as it meets the two above-mentioned requirements. Average capacity improvement of the NIMH cell over the high performance NICD cell is around 30%. Also, since it does not contain any heavy metal in its composition, it has minimal impact on the environment and thus is easily disposable.
Thus, there is now a demand for a battery charger which can charge both NICD and NIMH battery cells. A difficulty arises in that these batteries have different charging characteristics.
When charging a NICD or NIMH battery, the voltage and cell temperature characteristics take the form as illustrated in FIGS. 5 and 6. As illustrated in FIG. 5, the battery voltage increases slowly for 90% of the battery capacity during charging, but starts to climb steeply after that from point P1 in FIG. 5. The voltage then levels off at point P3 and, for a NICD battery, then starts to decline from point P3 to point P4. This is shown by curve b in FIG. 5. However, for a NIMH battery, the voltage tends to flatten out, from points P3 to P5 in curve a of FIG. 5.
FIG. 6 illustrates the temperature curve. The temperature increases gradually for 90% of the capacity (with a steeper increase for the NIMH battery) and increases more steeply near to the point of full charge, points P6 to P7 in curve c representing a NICD battery and points P8 to P9 in curve d representing a NIMH battery.
It is currently known to detect the end of battery charging for NICD batteries using a negative dV detection method, namely to detect the drop between P3 and P4 in curve b of FIG. 5. It is further known to use a dT/dt or peak voltage detection method for NIMH batteries. The dT/dt detection method looks at the change between points P6 and P7 on the NICD curve and points P8 and P9 on the NIMH curve. The peak voltage method detects the peak value P3 of FIG. 5. Thus, one difficulty with existing battery chargers is that it is necessary to determine what type of battery is being charged and to select the end of charge detection method accordingly.
Moreover, each of the existing methods of detection suffers from disadvantages.
For NICD batteries, if -dV detection is used, the probability of false detection and premature charge termination is high due to the abundance of switching and other noises present on the battery voltage during operation of the charger.
For NIMH batteries, if only dT/dt is used, they are normally not provided with a full charge (100%). Consequently, a lower current trickle charge which may require to be supplied for three to four hours, needs to be provided in order to ensure full charge.
If only zero dV/dt (or peak voltage) is used for detection for NIMH batteries, false detection is also common as the major portion of the charging curve (from 10% to 90% in FIG. 5) is also relatively flat, depending on the charging rate.
The invention seeks to provide a versatile battery charger which can be used for both NICD and NIMH batteries.